The present invention relates to high frequency solid state devices, and more particularly to biasing techniques for solid state power devices, for example, IMPATT diodes employed in RF circuits.
Power is supplied to solid state power devices generally in the form of a direct current or voltage through what is commonly referred to as the bias circuit. The dc bias circuit provides the energy that is converted from dc to RF by the solid state device, typically an IMPATT diode. The converted energy is coupled to a load through an RF circuit.
The common practice has been to bias IMPATT diodes with a constant current (CC) bias, i.e., a bias circuit adapted to supply a constant current to the device, even as the device voltage varies in operation. Such variations may be due, for example, to changes in temperature, RF circuit impedance, frequency or power of the RF input signal. Since the current is held constant, there is no adjustment in the bias to compensate for the changes in voltage. If the circuit performance was originally optimized for one condition, it would be degraded with the change in voltage. U.S. Pat. Nos. 4,359,700 and 4,328,470 are understood to disclose examples of constant current bias circuits.
The IMPATT device with a CC bias is generally operated as an injection-locked oscillator, whose performance is typified by high stage gain and narrow bandwidth. Bandwidths of IMPATT diode circuits for X band operation have generally not exceeded 1%.
The reference "Linear High Power IMPATT Amplifiers Using Constant Voltage Bias," J. W. McClymonds, G. C. Dalman and C. A. Lee, Proceedings of the Seventh Biennial Cornell Device Conference, pp. 349-359 (1979) discusses the use of constant voltage (CV) biasing as a way to improve the linear gain of IMPATT circuits. The change from CC to CV biasing results in correspondingly different RF circuit performance. The studies at Cornell University are believed to have demonstrated IMPATT operation with bandwidths of 20% or more using a low gain (3 to 4 db) reflection amplifier with CV biasing.
Thus, until very recently, IMPATT amplifiers have been biased using CC power supplies as were conventionally used with oscillators. In addition to performance improvements resulting from CV biasing of IMPATT amplifiers, the relative simplicity of CV pulser design over CC circuits may result in a considerable cost, weight and volume advantage over conventional CC pulsers. To achieve high combining efficiencies, however, IMPATT radar transmitters usually demand gains of 10 db or better, necessitating the use of injection-locked oscillators rather than lower gain amplifiers.
The CC and CV bias circuits can be characterized as direct bias compensation techniques with respective infinite and zero impedances for the bias circuit. These two known biasing techniques are illustrated by the conceptual bias circuits of FIG. 1(a), a CC bias circuit, and FIG. 1(b), a CV circuit. FIG. 2 plots the bias voltage V.sub.B applied to the diode against the bias current I.sub.B. The diode device lines 1 and 2 represent the IV characteristics of the IMPATT diode for two distinct operating temperatures. The intersection of the constant current line CC and device line 1 denotes an initial device operating point P1, indicating the initial bias voltage and current for the diode. For a device biased by a CV bias circuit, the line "CV" in FIG. 2 denotes the voltage level supplied by the bias circuit for the same initial operating point P1.
If for some reason, for example, with an increase in temperature, the IMPATT device line shifts to the device line 2, the new operating points for the CC and CV bias circuits would not be coincident, representing quite different device performance. The new CC operating point P2 is at a higher voltage than before, resulting in an increase in bias circuit power and, concomitantly, output power. Thus, use of CC biasing requires that the bias circuit be adapted to provide protection against thermal runaway, resulting in sacrifice of some available output power. This margin is provided by operating at somewhat less than the maximum available power. The new CV operating point P3, on the other hand, is at a lower current, resulting in a reduction in bias circuit power and operating power.
IMPATT diodes are often employed in RF combiner circuits to increase the available output power as, for example, described in U.S. Pat. No. 4,359,700. Generally, it is desirable to have the individual diode circuits in a combiner operate with substantially the same performance to maximize the output performance of the combiner circuit. Another drawback of CC or CV bias circuits is readily apparent from FIG. 2 if operating lines 1 and 2 are understood to represent two different IMPATT diodes. With the CC and CV bias circuits, two IMPATT diodes will have substantially different operating performance (power and frequency).
Moreover, the frequency, output power and functional bandwidth tend to shift as the temperature changes. For a CC biased RF combiner circuit, as the temperature increases, the output frequency is lowered and the average output power increases. These effects may result in a combiner circuit which fails to provide a required minimum power output at any one frequency in the combiner frequency bandwidth over the possible temperature range.
Another drawback of the CC bias circuit is that diode cold-start performance suffers from a slow heating effect, caused by the lower bias circuit power at lower temperatures.
It is known in the art to provide bias current compensation for IMPATT diodes using feedback circuits employing active devices, for example, U.S. Pat. No. 4,328,470. The desired goal of such feedback circuits is to provide bias circuit compensation as the operating characteristics of the device change. These feedback circuits suffer from decreased reliability resulting from the use of active devices, and are more complex and accordingly more expensive.
It is therefore an object of the present invention to provide a solid state device bias circuit adapted to increase the bandwidth of RF circuits in which the device is employed.
Another object of the invention is to provide a solid state device bias circuit adapted to result in increased circuit gain.
Yet another object of the invention is to provide a bias circuit for solid state devices which results in substantially linear gain over the circuit bandwidth.
A further object of the invention is to provide a bias circuit for solid state devices which is simplified and of greater reliability than prior art bias circuits.
Other objects of the invention are to provide a bias circuit for solid state RF devices which results in increased output power, reduced temperature sensitivity, elimination of thermal runaway and faster turn-on (cold start) characteristics.